* Christoph Hellwig:
> On Wed, May 19, 2010 at 11:33:27AM +0000, Florian Weimer wrote:
>> We've got a couple of rather large files, and with a cold cache,
>> reading the first 4K bytes of the file (e.g., just running
>> "head --bytes 4096" on it) takes ages, up to several minutes,
>> sometimes triggering the hang check timer.
>> I wonder if XFS reads the whole extent information into RAM when the
>> file is opened. Is this the case, at least on 2.6.26? Has this
>> changed in later versions, perhaps?
> Yes, XFS always reads in the extent map, and no this hasn't changed
Okay, defragmenting seems to improve things considerably. But it's
going to take a while: "extents before:5309152 after:13" *sigh*
Thanks for confirming my hunch. I don't think it's worth fixing this
in XFS. The database should call posix_fallocate() before flushing
its internal cache to the file in essentially random order, but it's
difficult to get upstream to implement this (the source code is a bit
hard to follow, unfortunately).
Florian Weimer <fweimer@xxxxxx>
BFK edv-consulting GmbH http://www.bfk.de/
Kriegsstraße 100 tel: +49-721-96201-1
D-76133 Karlsruhe fax: +49-721-96201-99